1. Field of the Invention
The present invention relates to a fuel feed system for an internal combustion engine.
2. Description of the Prior Art
A carburetor or an electronic fuel injection system (EFI) is used as a means for controlling the ratio of air and fuel supplied to an internal combustion engine. In either of them, the amount of air is determined as an initial value on an independent or priority basis, and the amount of fuel suitable therefor is determined dependently thereon. In such an air priority system (EFC), it is not easy to obtain compatibility between fuel economy and emission concentration. For instance, if the amount of air is changed in a substantially stepped manner by the operator (driver), there is a response delay on the part of fuel because fuel has a larger density than air. In a state of acceleration, the pressure difference between portions before and after a throttle valve is very large and much air flows in momentarily. In such a case, compensation is necessary to keep the mixture of air and fuel combustible within a combustion chamber. The inventors herein have already provided a fuel feed system of a fuel priority (engine air control) type, in which outputs from control elements, such as a fuel command potentiometer for detecting the degree of movement of an accelerator pedal, an air flow sensing device within an intake bore and a potentiometer connnected to a throttle valve actuator, and electric signals from compensation elements sensing engine cooling water tenperature, engine cylinder head temperature, atmospheric temperature, atmospheric pressure, fuel feed line pressure, etc., are put into a control unit and compared thereby with memories programmed in advance on the basis of the functional relations between the parameters of said control elements and compensation elements, the throttle valve being actuated on the basis of a required amount of air calculated from fuel flow input so as to give an optimum amount of air. In such a fuel priority system, a required air fuel ratio can be obtained with little delay both in the rise and fall of fuel, and the air fuel ratio can be selected easily and programmed. Particularly in urban areas in which motor vehicles are forced to repeat acceleration and deceleration frequently, the total fuel consumption is much reduced and emission control is facilitated. However, in said fuel priority system, if the throttle valve actuator or the control unit fails to operate properly from one cause or another, the engine will stop and the vehicle cannot run by itself.